Hello Dmitry
I received the Quasimodo board and all the needed parts.
I must congratulate you for your outstanding job. The board is very high quality and all the parts came in small bags with carefully printed tags.
It was a breeze to stuff the board and I have the rig up and running.
Thank you so much.
BTW. The rig works perfectly.... will post some scope pics latter 🙂
In-circuit Quasimodo?
Excuse me if this had been asked before but here goes: is there a way to run the Quasimodo snubber test in-circuit?
By this I mean without isolating the transformer from the circuit and shorting the primary, but run it as it is in the power supply circuit. Intuitively if one runs it while running the power supply at the same time the operating conditions may be even more precise than testing the transformer in isolation?
If this can be done then it will be wonderful as we don't need to remove the transformer from an existing equipment - in some cases can be quite difficulty, to retrofit the snubber.
Thanks.
Excuse me if this had been asked before but here goes: is there a way to run the Quasimodo snubber test in-circuit?
By this I mean without isolating the transformer from the circuit and shorting the primary, but run it as it is in the power supply circuit. Intuitively if one runs it while running the power supply at the same time the operating conditions may be even more precise than testing the transformer in isolation?
If this can be done then it will be wonderful as we don't need to remove the transformer from an existing equipment - in some cases can be quite difficulty, to retrofit the snubber.
Thanks.
What is wanted is a way to view the voltage across the transformer secondary while adjusting a potentiometer. This is especially easy if one of the secondary's terminals is connected straight to ground. It's not so straightforward if neither end of the secondary is ground -- as for example in a voltage doubler supply in valve equipment. How do you measure the voltage across the secondary? Where do you attach the ground clip of your oscilloscope probe?
Jim Hagerman {author of THIS influential paper} wrote in an email to me,
You may also encounter a (happy) situation in which the core losses at the resonant frequency are significant. This is seen in Quasimodo testing of some transformers: the R=Infinity waveform does not "ring like a bell" very much; instead it dies away quickly. And that's when you're smacking the bell with a great big steel sledgehammer (i.e. a MOSFET with Rds=0.006 ohms). If instead you lightly tap the bell with a bubble-wrapped plastic mallet (i.e. a soft recovery diode), it won't ring hardly at all. And you won't see any wiggles on your oscilloscope, so twirling the shaft of the snubber potentiometer will have no visible effect. Try to remind yourself this is a happy situation, a High Quality Problem To Have. You can't dial up an Optimum Snubber because the thing is already pretty well damped even without a snubber.
Jim Hagerman {author of THIS influential paper} wrote in an email to me,
I would just use a Tektronix 7A22 scope input and limit the LF bandwidth. The normal 120Hz operation of the circuit would then show the HF ringing.
If you've never seen a 7A22 differential amplifier module, this photo might help. It plugs into a Tektronix mainframe oscilloscope, replacing one of the vertical amplifier modules.You may also encounter a (happy) situation in which the core losses at the resonant frequency are significant. This is seen in Quasimodo testing of some transformers: the R=Infinity waveform does not "ring like a bell" very much; instead it dies away quickly. And that's when you're smacking the bell with a great big steel sledgehammer (i.e. a MOSFET with Rds=0.006 ohms). If instead you lightly tap the bell with a bubble-wrapped plastic mallet (i.e. a soft recovery diode), it won't ring hardly at all. And you won't see any wiggles on your oscilloscope, so twirling the shaft of the snubber potentiometer will have no visible effect. Try to remind yourself this is a happy situation, a High Quality Problem To Have. You can't dial up an Optimum Snubber because the thing is already pretty well damped even without a snubber.
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I have never done that myself, nor have I ever heard of anyone else using Quasimodo to determine snubbers for loudspeakers.
I recommend that you create a new discussion thread in the loudspeakers forum, with a provocative title that will generate many replies. Possibly something like:
- How can I determine snubbers for loudspeakers? Use a pulse generator like Quasimodo?
Snubbers for speakers is a different anilmal.
A R+C parallel to the speaker has two uses.
a.)
One can be to load the cable feeding the speaker so that VHF interference sees the R part of the loading and absorbs the energy of the interference. this attenuates any reflected VHF and in turn helps prevent resonances in the cable being fed back into the source/amplifier.
b.)
one can load the speaker driver so that the rising impedance seen by the source due to inductance of the driver voice coil, is effectively counteracted by the added R+C, to give a near constant impedance of the combined speaker driver||R+C.
This allows for a simpler selection of crossover filter components to give a predictable roll off at the desired crossover frequency.
Either a, or b, or both can be added to the speaker.
But neither would be termed "snubbers" as in quasimodo parlance.
A R+C parallel to the speaker has two uses.
a.)
One can be to load the cable feeding the speaker so that VHF interference sees the R part of the loading and absorbs the energy of the interference. this attenuates any reflected VHF and in turn helps prevent resonances in the cable being fed back into the source/amplifier.
b.)
one can load the speaker driver so that the rising impedance seen by the source due to inductance of the driver voice coil, is effectively counteracted by the added R+C, to give a near constant impedance of the combined speaker driver||R+C.
This allows for a simpler selection of crossover filter components to give a predictable roll off at the desired crossover frequency.
Either a, or b, or both can be added to the speaker.
But neither would be termed "snubbers" as in quasimodo parlance.
This is what I was talking about.
Thank you so much Andrew.
I have been experiemnting with some woofers and it seems to work (at least in the scope).... I will implement the zobels on the woofers and measure them with Arta to see if the impedance curve gets more linear 🙂
BTW.... Must thank you mark for the splendid text on your pdf.... reading it evaporated all my doubts about TX snubbering.....
Mark, sorry for very well chewed question, but one more time please....
Should we trust our eyes to bring ringging to the min level without overdumping or we do need to calculate Zeta evry time? If no calculation needed, then I (probably others too) would like to see more examples (pistures) of overdumping which we're tring to avoid.
Thsank you.
Should we trust our eyes to bring ringging to the min level without overdumping or we do need to calculate Zeta evry time? If no calculation needed, then I (probably others too) would like to see more examples (pistures) of overdumping which we're tring to avoid.
Thsank you.
While more examples could be useful, you might also consider cross-checking the results obtained when dialing the Rs trimmer until a damped pattern is obtained, by using a method that combines the ringer with a calculation formula.
I'm enclosing an Excel sheet which summarizes my experience from using the Quasimodo ringer on ten different transformers. The sheet contains also the formula for calculating an optimal snubber proposed in the paper by Hagerman (referenced by Mark in an earlier post). In order to apply the formula, one must know the ringing frequency with only Cx in place, and (optionally) the resistance of the secondary winding. The frequency can be determined from the free oscillatory response of the secondary when ringed by Quasimodo. If one is lucky to have a digital scope with cursors, it can be read off the screen directly; otherwise it can be obtained by counting the graticules on the screen to determine the signal period.
Having obtained the resonant frequency, simply enter it into the corresponding green cell in the sheet (E16 in my case), enter the secondary winding resistance into cell D16, and the value of the snubber resistor for the damping factor Zeta given in cell G3 (currently Zeta=0.707) pops up for you in cell J16 of the sheet. The value of Zeta in the sheet can be changed at will.
A value Zeta=1.0 has been recommended by Mark several times, and it one that makes sense. The reason why my table was done with Zeta=0.707 is that at the beginning I tested a couple of transformes with this value, and was lazy to redo the measurements with Zeta=1.0 later.
As it is, the last line in the sheet merely repeats the case in the row above (the Era transformer) for demonstration purposes.
Apparently, the agreement between the Rs values obtained by pattern matching and those calculated on the basis of the natural (resonant) frequency is good in most of the cases. There always will be difficult transformers, and especially in such cases it is not bad to have two methods for determining the snubber resistance.
Regards
Hagerman's paper is Reference #1 in the Quasimodo design note. In the .pdf file it's a hyperlink, so when you're reading the design note you can click on reference 1 and boom! your browser opens up Hagerman's paper. This computer stuff is magical.
(It's NOT a hyperlink in the screen capture image below; this image is merely a copy of the pixels, but not the magic.)
_
Attachments
Mark something I was wondering.
Have you tried ringing/snubbing any tube rectifiers? Like EZ81 or something?
Have you tried ringing/snubbing any tube rectifiers? Like EZ81 or something?
Dimkasta just so you know this thread is about supressing transformer ringings. Stuff with windings. Not rectifiers thats a completely different story. Mark might have thought about that to nevertheless🙂
Transformers, well coils in any form has a self oscillation tendency in specific frequencies. Quasimodo is a platform for identifying where those are and what you use to kill that.
Diodes produces other unwanted artifacts, where you can use so called snubbers to elliminate. But that is not what quasimodo is built for.
I see it as a cost vs. benefit decision. The cost of snubbing a transformer secondary when tube rectifiers are used, is simply {2 capacitors + 1 resistor}. If you need 1600VDC capacitors, they might cost 2,10 € each (link to se.mouser.com).
There are two benefits in my opinion. The first benefit is pragmatic: you will know, with perfect certainty, that the transformer secondary is well-damped and cannot possibly oscillate. No matter what the stimulus, your optimum snubber damps any and all potential ringing. The second benefit is psychological: you will know that your DIY design does everything humanly possible to eliminate transformer secondary oscillations. You have left no stone unturned in your quest for perfection, just like an ultra High End circuit designer. You can be proud of your design work.
Please notice that I am explicitly talking about "snubbing a transformer secondary". If you want to "snub a rectifier", whatever that means, I don't think Quasimodo will help.
There are two benefits in my opinion. The first benefit is pragmatic: you will know, with perfect certainty, that the transformer secondary is well-damped and cannot possibly oscillate. No matter what the stimulus, your optimum snubber damps any and all potential ringing. The second benefit is psychological: you will know that your DIY design does everything humanly possible to eliminate transformer secondary oscillations. You have left no stone unturned in your quest for perfection, just like an ultra High End circuit designer. You can be proud of your design work.
Please notice that I am explicitly talking about "snubbing a transformer secondary". If you want to "snub a rectifier", whatever that means, I don't think Quasimodo will help.